AssholeDesign

You don't have to give Microsoft the key (unless you want the "backup" option) but the OS has to have the key locally while it's running in order to be able to read the data on the drive (and also write new data).
In typical usage The TPM holds the key, but it's the OS that generated the key and encrypted the drive in the first place. I don't know the technical details but the TPM recognises the OS install that programmed it and will only automatically unlock and provide the key for that. If you change it by swapping the drive or booting to a different device it remains locked and any alternative OS requires the key to be entered manually.